The present invention relates to a method for producing a clean cold strip.
In cold rolling of a steel strip, cold rolling mill lubricants such as mineral oil, beef tallow, synthetic oil, or the like are used to ensure cooling and lubrication of the mill rolls. On the surface of a strip after cold rolling, there are usually attached stains of the cold rolling mill lubricant used in the rolling and metal dust produced by abrasion during the rolling operation. When the strip coil with such stains remaining thereon is annealed, these stains are charred and adhered to the surface of the strip as surface smudges, thereby considerably spoiling the surface appearance to lower the commercial value and considerably damaging the coating and the resistance to corrosion.
When using such cold strip as a raw plate for plating, the smudges remaining firmly on the surface of the strip are not removed by pretreatment for plating, causing serious defects on the surface of the product preventing a firm coating from being formed, whereby the product is spoiled in appearance and considerably reduced in resistance to corrosion. Further, when using such cold strip as a raw plate for coating, a sound surfacing coat of such material as phosphate is not formed, thereby reducing considerably the property of the plate for coating.
For this reason, when high cleanness in the surface is required, a cold rolled strip has usually been cleaned in the surface by a cleaning line before annealing. However, such cleaning line requires additional energy such as electricity and water steam, detergent such as alkali, expendables such as brushes, and labor, thereby increasing the total production cost.
In cold rolling or annealing, accordingly, there have been heretofore practised various methods for cleaning the surface of a strip without using a cleaning line. However, none of them have been satisfactory in cost or in cleaning effect.
Some of the methods heretofore employed in cold rolling for cleaning the surface of the strip are as follows:
(1) Detergent Method
According to this method, water or a detergent is injected generally at the pressure of 10 Kg/cm.sup.2 or lower onto the surface of the strip at the last stand or pass to purge the surface of the strip from the oil and metal dust.
However, use of water is liable to cause surface defects such as rust and water stain, to worsen the rolling mill lubrication, to lower the rolling efficiency, to make the rolling of thin articles difficult, and to produce a large quantity of metal dust causing smudges.
Further, use of a detergent is too costly in proportion to effects obtained thereby and has such additional problems that decrease in rolling mill lubrication and necessity for disposal of waste fluid.
(2) High Pressure Detergent Injection Method
In this method, a detergent at high pressure and at high temperature is injected onto the surface of the strip in the final rolling. A typical example of this method is disclosed as "Clean Cold Rolling Method" in Japanese Patent Publication No. 19682/80. According to this method, while the cleaning efficiency is greatly improved, use of water and/or detergent causes rust and decrease in lubricating property, leading to production of metal dust by abrasion, resulting in such problems as smudges after annealing.
(3) Mill Clean Method
In this method, a rolling mill lubricant emulsion used for cooling and lubrication of the rolling mill is improved in annealing evaporative property. According to this method, it is intended that the rolling mill lubricant remaining on the surface of the strip will not be charred during annealing but will be decomposed and evaporated to prevent the property of the strip surface from being worsened. In this method, however, the cleaning effect obtained thereby is small and the lubricating property has to be sacrificed because it is difficult to obtain a rolling mill lubricant emulsion having both the lubricating property and the evaporating property during annealing.
(4) Annealing Gas Cleaning Method
In this method, steam is mixed with an atmospheric gas consisting mainly of N.sub.2 gas and H.sub.2 gas to make the furnace gas decarburizing, to thereby remove the residual carbon produced during annealing by the oil attached on the surface of the strip. Typical examples of this method are disclosed by, for example, Japanese Patent Publications Nos. 23817/71, 43401/73 and 18125/77. According to this method, although a gas cleaning effect is recognized, when the amount of the oil and metal dust attached on the surface of the material to be annealed, that is the cold strip, is a certain value, for example, 100 mg/m.sup.2 or above, the effect is limited. Conversely, when the amount of the steam to be mixed is increased, while the gas cleaning effect is increased there is caused local decarburization in the strip, resulting in defects in the quality of the product. In detail, in box type coil tight annealing, the local decarburization in the inner and the outer peripheral edges of the strip coil necessitates it that the amount of the steam to be mixed for gas cleaning be limited, resulting in a failure to provide the desired cleaning effect.